The production of styrenic polymer beads by an aqueous suspension process is a well known and commercial practice. Such polymerization may be effected by the suspension polymerization of a styrenic monomer in an aqueous medium with an oil-soluble polymerization catalyst, using a suspending system comprised of a finely divided difficultly water-soluble inorganic phosphate, such as tricalcium phosphate, in conjunction with an anionic surface-active agent, such as sodium dodecylbenzene sulfonate, as a modifier. Such suspension polymerization systems are described, for example, in Grim patent, U.S. Pat. No. 2,673,194.
In the use of known suspension polymerization systems, such as those taught in the Grim patent, however, styrenic polymer beads are produced which vary in size. The range of bead size can vary widely, dependent upon the type and intensity of mixing and the particular suspension system selected. Although the average bead size can be controlled, to an extent, by adjustment of the involved variables, the bead size distribution will remain broader than that required, for example, by the expandable polystyrene bead foam industry.
Attempts have been made to utilize off-size beads by dissolving them in styrenic monomer and subjecting the solution to a suspension polymerization with the addition of an oil-soluble catalyst; by suspending small beads in water and effecting suspension polymerization, with a catalyst, after gradual addition of styrenic monomer to the suspension system; or by suspending small polystyrene beads in water and adding styrene monomer in the presence of a catalyst therefor plus a polymerization retarder, as described in British Pat. No. 1,416,405. As described in said British patent, the use of a polymerization retarder is suggested so as to form large beads from the small styrene beads without the production of significant amounts of fine powdery polymeric particles. Even with the practice of the process described in said British patent, however, powdery polystyrene particles are formed in an amount of about 0.3 or more percent based upon the weight of the polymer beads produced.
An object of the present invention is to provide a process for producing styrenic polymer beads of a preferred bead size from styrenic polymer beads of a smaller size.
Another object of the present invention is to produce styrenic polymer beads of a preferred size from styrenic polymer beads of a smaller size while using conventional catalyst systems for styrene polymerization.
A further object of the present invention is to provide a process for producing styrenic polymer beads of a preferred size from styrenic polymer beads of smaller size while preventing the formation of any significant amount of fine powdery polymer particles.
An additional object of the present invention is to form styrenic coatings on styrenic polymer beads over a broad range of coating sizes and bead sizes.
A further additional object of the present invention is to produce styrenic polymer beads having a narrow bead size distribution by the formation of styrenic coatings thereon.